The present invention relates to a fuel assembly for a nuclear reactor of boiling water type containing a plurality of vertical fuel rods, which are arranged between a bottom-tie tie plate and a top-tie plate in a surrounding vertical casing part and which extend through a number of spacers which are arranged in spaced relationship in the vertical direction and which together with the bottom-tie plate and the top-tie plate retain the fuel rods in a spaced relationship in a lateral direction. Each fuel rod in the fuel assembly comprises a cladding tube containing a nuclear fuel material and the fuel assembly is formed with an inlet for water for conducting water in through the bottom-tie plate, through the space between the fuel rods in the vertical casing part, and out through the top-tie plate.
In the operation of a nuclear reactor with a core containing several fuel assemblies of the above-mentioned kind, there may be a risk of so-called dryout occurring, that is to say the disappearance or breaking down in limited regions of the water film which is present on the surface of each fuel rod, which locally leads to a considerably deteriorated heat transfer between the fuel rod and the water conducted through the fuel assembly, resulting in a greatly increased wall temperature of the fuel rod. The increased wall temperature may lead to damage with serious consequences arising on the fuel rod.